Surgical microscopy system and method for performing eye surgery

ABSTRACT

A surgical microscopy system comprises microscopy optics for generating an image of an eye under surgery. A pattern generator generates a pattern to be superimposed with the image. An eye-tracker is provided for tracking a position of the superimposed pattern with respect to the image in case of a movement of the eye. The superimposed pattern comprises pattern elements that are equally distributed on first and second circles of different sizes, in order to give assistance when placing a suture during a corneal transplant. The superimposed pattern may also provide an assistance for orientating a toric intra-ocular lens.

This application is a Divisional of co-pending application Ser. No.11/259,556, filed on Oct. 26, 2005, which claims priority to ApplicationNo. 10 2004 052 031.3 filed in Germany on Oct. 26, 2004, and ApplicationNo. 10 2004 055 683.0 filed in Germany on Nov. 18, 2004, the entirecontents of which are hereby incorporated by reference into the presentapplication and for which priority is claimed under 35 U.S.C. §120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surgical microscopy system forperforming eye surgery. The invention also relates to a method ofperforming a surgery on the eye. Further, the invention relates to amethod of preparing a surgery on the eye.

2. Brief Description of Related Art

An example of an eye surgery is a corneal transplantation. In suchsurgery, an original cornea is removed within a circular region, and acorresponding circular graft is inserted to replace the removed cornea.The inserted graft is sewn to the remaining natural cornea. Acorresponding suture should be set with regular stitches, in order tonot disturb a symmetry of the eye. In particular for a relativelyinexperienced surgeon it is difficult to place the stitches of thesuture with a sufficient regularity.

Another example of an eye surgery is a cataract surgery. In a cataractsurgery, the natural lens of the human eye, in which a cataract hasdeveloped, is replaced by an artificial lens. The surgery is amicro-surgical treatment performed by an operating surgeon using asurgical microscope. An opening in the lens capsule is achieved withinthe inner rim of the iris by providing an incision through the sclera orcornea without injuring the iris. The body's natural lens is removed viathis incision by breaking the natural lens into pieces using ultrasonicenergy and removing the pieces using a suction device. The artificiallens is then inserted into the lens capsule through the opening.

US 2004/0102799 A1 discloses a surgical microscopy system projecting aring pattern into a microscopic image of the eye to be operated upon.The projected ring pattern helps the operating surgeon to place acorrect incision into the lens capsule. The operating surgeon can usethe projected ring pattern as a guide for performing the cut. Thereby itis possible to precisely adjust a diameter of the incision relative adiameter of the artificial lens that is to be inserted. This reducesadverse long-term consequences of the treatment. Using the surgicalmicroscopy system projecting the ring pattern, long-term consequencesare somewhat reduced as compared to the conventional treatment in whichthe operating surgeon places the incision merely by visual judgment.Despite improvements in the conventional eye-surgery microscopy system,expectations are still high regarding reduction of long-termconsequences and further improvements appear to be possible.

SUMMARY OF THE INVENTION

The present invention has been accomplished taking the above problemsinto consideration.

Embodiments of the present invention provide a surgical microscopysystem and method for performing a surgery on an eye of a patient.

According to an embodiment of the invention, an eye surgery microscopysystem comprises microscopy optics for generating an image of an eyearranged in one object plane of the microscopy optics, and a patterngenerator for the generation of a pattern to be superimposed on theimage.

The pattern can be for example a circle pattern that can serve as aguiding line for the operating surgeon for placing an incision in thelens capsule of the operated eye. However, the pattern can also be apattern, other than a circle pattern that can assist the operatingsurgeon in another respect.

According to an embodiment of the present invention, the surgicalmicroscopy system comprises an eye-tracker in order to detect a positionof the operated eye in the image, and to adapt a position of thesuperimposed pattern relative to the image in dependence of the detectedposition of the eye in the image. In particular, the position of thepattern in the image can be fixedly maintained relative to the image ofthe eye.

It has been observed that a force exerted by a surgical tool, such as ascalpel or a shear, may shift an eyeball within an eye socket of thepatient relative to the microscopy optics in an unforeseeable way, andthus relative to the object plane of the microscopy optics. In thiscase, if a cut is produced along a guiding line of a staticallysuperimposed ring pattern, the cut may have a shape that does notcorrespond to a desired shape.

By using the eye-tracker that produces a position signal in dependenceof the detected position of the eye, and by configuring the patterngenerator such that the pattern generator changes the position of thepattern in the image in dependence of the position signal, it ispossible to achieve a more precise cuts and other manipulations on theeye under surgery. Still, the representation of the eye willcontinuously be displaced within the image due to the application offorces exerted by the surgical tools. However, due to the function ofthe eye-tracker in combination with the pattern generator, thesuperimposed pattern will be displaced in correspondence with thedisplacements of the image of the eye, and the surgeon can achieve abetter success by tolerating the permanent displacements of the patternrelative to the image.

According to an exemplary embodiment of the invention, the eye-trackercomprises an image processor that configured to detect a center of acontiguous dark region in the image and to provide the position signalin such that it represents the center of the dark region. In thisrespect, it may be of advantage to illuminate the operated eye such thatan inside of the pupil appears as a dark region. In such treatments, aninner rim of the pupil is usually clearly defined and is maintained in asubstantially constant position relative to a center of the eye. Thus,the centre of the eye is typically well detectable by the above imageprocessor even during difficult operation conditions.

According to a further embodiment of the present invention, a surgicalmicroscopy system comprises a microscopy optics for generating an imageof an object plane and a pattern generator for generating a patternsuperimposed with the image. The pattern generator generates thesuperimposed pattern such that the pattern comprises two groups ofpattern elements. Each group of pattern elements comprises pluralpattern elements. The pattern elements of the first group aredistributed on a first circle, and the pattern elements of the secondgroup are distributed on a second circle. The second circle iscompletely arranged within the first circle. In this respect, said firstand the second circles can have a substantially common centre. In thecontext of the present application, the centers of the first and secondcircles are substantially the same if a between the centre of the firstcircle and the centre of the second circle is smaller than 0.15 times,and in particular smaller than 0.07 times a diameter of the secondcircle.

When performing an implantation of a cornea, the operating surgeon canuse the superimposed pattern elements as reference points in order toprovide stitches for a suture for holding the implant at locations wherethe pattern elements appear in the image. Thus, when placing the suture,the operating surgeon no longer has to rely on his estimation by sightthat is free and subject to deceptions. Rather, a desired stitchingpattern can be entered into the pattern generator via an interfacebefore the execution of the treatment as data representing the stitchpattern. This is performed in a way such that the pattern generatordisplays the pattern elements, which can be used by the operatingsurgeon as auxiliary markings, at the respective positions in themicroscopic image.

The interface may comprise a first interface portion for inputtingdiameters of the first and second circles. Furthermore, the interfacemay comprise a second interface portion for inputting a number of thepattern elements of the first group or the number of the patternelements of the second group. Further, the interface may comprise athird interface portion for inputting a circumferential position of thepattern elements of the first group relative to a circumferentialposition of the pattern elements of the second group. In this waydifferent suture types, such as a zigzag type suture and a suture thatcomprises a multiplicity of stitches extending in a radial direction,can be easily predefined.

According to an exemplary embodiment, the diameters of the first andsecond circles differ by a ratio from 0.5 to 0.8. According to a furtherexemplary embodiment, a dimension of the pattern elements in the imageis large enough to be easily noticed by an operating surgeon, but at thesame time small enough to define the place of the stitch that is to beperformed with a sufficient precision.

The pattern elements can exhibit an arbitrary compact shape, such as ashape of a circle, a filled circle, a rhombus or similar shapes. Furtherthe pattern elements my also be represented by shapes such as crosses,stars and similar shapes.

According to a further embodiment of the invention, a method ofpreparing of a surgical treatment of the eye comprises: generating amicroscopic image using a microscopy optics such that at least a portionof an iris or a portion of a limbus of the eye under surgery is visiblein the image, and generating a pattern superimposed with the microscopicimage, wherein the pattern comprises a first group of plural patternelements disposed on a first circle, and a second group of pluralpattern elements disposed on a second circle which is located within aninterior of the first circle.

According to a further embodiment of the invention, a surgicalmicroscopy system comprises a microscopy optics for generating an imageof an object plane of the microscopy optics, and a pattern generator forgenerating of a pattern to be superimposed with the image.

The pattern generator is configured to generate a first partial patternessentially extending along a ring, and a second partial patternessentially extending along a straight line, wherein the straight linehas two intersections with the ring, and wherein an orientation of thestraight line is changeable around a centre of the ring. According to anexemplary embodiment, the pattern comprises a TABO-pattern displaying anangular scale. The TABO-pattern is well known to the person skilled inthe art as a pattern allowing measurement of angles at the eye.

Such type of pattern is particularly helpful with regard to inserting oftoric intraocular lenses (IOL) into an eye of a patient. When performinga conventional method, the operating surgeon was dependent on his freeestimation by sight. Therefore, it has been difficult to ensure adesired orientation of the toric intraocular lens relative to the eye.Since the partial pattern extending along the straight line issuperimposed with the microscopic image by using the pattern generator,it possible for the operating surgeon to use this partial pattern as anassistance in orientating the intraocular lens. In this respect theintraocular lens itself can exhibit markings or design characteristics.The operating surgeon can shift the intraocular lens relative to therecognized image such that the markings and/or design characteristicsprovided on the intraocular lens coincide or are registered with thepartial pattern extending along the straight line. The partial patternextending along the ring can be useful in order to correctly positionthe partial pattern extending along the straight line relative to theimage. This can be achieved by shifting the microscopy optics relativeto the eye. It is, however, also possible to input positioning signalsto the pattern generator via an interface, to instruct the patterngenerator to shift the generated pattern in the image as a wholeaccording to the entered signals. When using the eye-tracker it may bepossible to not generate the first partial pattern extending along thering during time intervals, to not generate the first partial pattern atall.

According to a further exemplary embodiment, the surgical microscopysystem comprises an interface for inputting an orientation and/or achange of the orientation of the straight line in the image.

In accordance with a further embodiment of the invention, a method ofpreparing an implantation of a toric intra-ocular lens comprises:generating a microscopic image, using a microscopy optics, such that atleast a portion of an iris or a portion of a limbus of an eye to beoperated upon is visible in the image, superimposing onto the image asecond partial pattern essentially extending along a straight line, andadjusting an orientation of the second partial pattern.

According to an embodiment, the orienting of the straight line isperformed by firstly changing the orientation of the straight such thatthe mark coincides with a mark applied to the operated eye, wherein suchmark has been applied to the eye before starting the treatment of theeye. Such mark may represent a predetermined orientation, such as avertical orientation. Since the eye can be shifted and also rotated inthe eye socket during the treatment, this mark applied to the eyeconstitutes a reference for an orientation of the toric intraocularlens. After the straight line has been oriented such that it coincideswith the mark applied to the eye, the orientation of the straight lineis then changed by a predetermined angular amount that corresponds to adesired orientation of the intra-ocular lens with respect to the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing as well as other advantageous features of the inventionwill be more apparent from the following detailed description ofexemplary embodiments of the invention with reference to theaccompanying drawings. It is noted that not all possible embodiments ofthe present invention necessarily exhibit each and every, or any, of theadvantages identified herein.

FIG. 1 is a schematic illustration of a surgical microscopy systemaccording to an embodiment of the present invention;

FIG. 2 is a schematic representation of an image generated by themicroscopy system of FIG. 1;

FIG. 3 is a schematic representation of an image corresponding to theimage of FIG. 2, wherein a marker pattern for assisting in setting asuture during a corneal transplantation is superimposed with the image;

FIG. 4 is a schematic representation of an image corresponding to theimage of FIG. 2, wherein a marker pattern for assisting in setting asuture during a corneal transplantation is superimposed with the image;

FIG. 5 is a schematic representation of an image generated by themicroscopy system of FIG. 1 before insertion of an intra-ocular lens;

FIG. 6 is a schematic representation of an image generated by themicroscopy system corresponding to the representation of FIG. 5, whereinan auxiliary pattern is displayed for orienting the intraocular lens inan intermediate step; and

FIG. 7 is a schematic representation of an image generated by themicroscopy system corresponding to the representation of FIG. 5, whereinthe auxiliary pattern is displayed for orienting the intraocular lens ina further intermediate step.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments described below, components that are alikein function and structure are designated as far as possible by alikereference numerals. Therefore, to understand the features of theindividual components of a specific embodiment, the descriptions ofother embodiments and of the summary of the invention should be referredto.

FIG. 1 schematically illustrates a surgical microscopy system 1. Thesurgical microscopy system 1 comprises a housing body 3 for mountingmicroscopy optics 5. The microscopy optics 5 comprises an objective lens7 transforming an object-side divergent beam 11 emanating from an objectplane 9 of the objective lens 7 into an image-side parallel beam 13. Apair of zoom systems 15, each comprising groups 19 and 21 of lenseswhich are displaceable in a direction of an optical axis 17 of theobjective lens 7, supply two partial beams 23 and 24 of the image-sideparallel beam 13 to oculars 25 and 26. The operating surgeon can lookwith his left and right eyes into the oculars 25,26 in order to perceivean image of the object plane 9.

For performing an eye surgery, the operating surgeon arranges themicroscopy system 1 in front of an eye 29 of a patient. For performing acataract surgery, the operating surgeon first creates an entrance to alens capsule of the eye, for example by accomplishing appropriateincisions at a sclera or a cornea of the eye 29. Then, preparatory stepsfor providing an incision at the lens capsule are performed. For thispurpose, the microscopy system 1 comprises a controller 31, such as apersonal computer, that controls a motor 33 for displacing the groups oflenses 19, 21 of the zoom systems 15 and, thus, for changing amagnification of the microscopy optics 5. The controller 31 receivescommands for controlling the motor 33 from a switch desk 35 havingpush-buttons 37, 38 and 39 that are operatable by a foot of theoperating surgeon or a person preparing the incision at the lenscapsule. If the push-button 37 is pressed, the controller 31 controlsthe motor 33 such that the magnification of the microscopy optics 35 isincreased. Similarly, when pressing the push-button 38, themagnification of the microscopy optics is decreased.

The microscopy system 1 further comprises a semi-permeable mirror 41disposed in the beam 23 for supplying a portion 43 of the beam 23 to acamera chip 47 via an adapting optics 45 such that an image of theobject plane 9 is generated on the camera chip 47. The controller 31reads out images detected by the camera chip 47 and displays the imageson a display 49. Thus the same image of the object plane 9 as it isperceived by a viewer looking into the ocular 25 is visible on thedisplay 49.

This image further displayed on a head mounted display 50 that can becarried by a person, such as the operating surgeon, on his head. Thus,operating surgeon has three different possibilities of watching theobject plane. A first possibility of looking into the oculars, a secondpossibility of watching the operation field on the display 49 and athird possibility of watching the operation field by using the headmounted display 50. In the case of using the head mounted display 50 itis advantageous to provide a camera system as it is represented in FIG.1 by the components 41, 43, 45 and 47 for the left beam path in bothstereoscopic optical beam paths of the microscopy system 1. Thus, thetwo cameras can generate a stereoscopic image of the object plane 9, andthis image can be displayed in a stereoscopic way by the head mounteddisplay 50.

The controller 31 is part of an eye-tracker. For implementing theeye-tracker, the controller 31 comprises a software module 63 foranalyzing the images received from the camera 47. A representativeillustration of such image 101 is shown in FIG. 2. Lids 103, a sclera105, an outer rim 107 of a pupil 109 and an inner rim 111 of the pupil109 can be recognized in the image 101. By using an appropriateillumination of the eye, the sclera 105 appears as a white area, theiris 109 appears as an area having a colour corresponding to the eyecolour of the patient, and the pupil 113 positioned within the inner rim111 of the iris 109 appears as a dark or black area. The software module63 analyzes the image with respect to its brightness values and providesa threshold filter with regard to brightness. By applying the filter,only those regions of the image, which have a brightness below athreshold value, are maintained in the image.

This typically results in a contiguous dark region 113 of the pupil andfurther smaller details of the image 101, such as lashes or similar.Thereafter, the image processing software determines a largestcontiguous dark region which will correspond to the pupil 113 inpractice. A geometrical centre of gravity of the largest contiguous darkregion is determined. The reference sign 115 in FIG. 2 designates suchgeometrical center of gravity. Thus it is possible to determine thecenter 115 of the pupil in the coordinates of the image 101 by using theeye-tracker.

The microscopy system 1 further comprises a projector 55 having adisplay 57, such as a LCD display, projection optics 59 and asemi-permeable mirror 61. The semi-permeable mirror 61 is arranged inthe beam 24 and superimposes a projected pattern displayed by thedisplay 57 and optics 59 with the beam 24 such that it is perceived insuperposition with the image of the object plane 9 when looking into theocular 26 or the head mounted display 50 or when looking on the display49. The pattern displayed by the display 57 is generated by a patterngenerator 64 that is part of the controller 31, wherein the controller31 further superimposes this pattern onto the image that is shown on thedisplay 49. In this respect, the controller generated the patternsuperimposed with the image in a size that automatically scales with theadjusted magnification of the zoom systems 15. Thus, if the user changesthe magnification of the zoom system 15 by a certain factor, thecontroller 31 also changes the size of the superimposed pattern by thesame factor.

FIG. 3 shows the microscopic image 101 of the object plane (compare withFIG. 2), wherein a pattern generated by the pattern generator 64 issuperimposed with the image. The pattern comprises of a plurality ofsmall rings 121 as outer pattern elements and a plurality of small rings123 as inner pattern elements. The small rings 121 are located on acircle 125. The circle 125 does not necessarily have to be displayed inthe image 101. The small rings 123 are located on a further circle 127that is located within the circle 125. It is also not necessary that thecircle 127 is displayed in the image 101. Broken line 129 designates aborder between a natural cornea of the eye that is located outside ofthe circle 129, and a cornea implant, that is located in an interior ofthe circle 129. A zigzag type suture 131 fixes the implant to thecornea. Straight sections 133 of a thread providing the suture extendbetween the rings 121 and 123. The rings 121 and 123 that aresuperimposed in the image 101 serve the operating surgeon as positioningaids for stitches fixing the thread 133. In the described example, thesuture comprises thirty-two stitches corresponding to sixteen rings 121which are equally distributed on the outer circle 125, and sixteen rings123 which are equally distributed on the inner circle 127. In theillustrated example, a diameter of the circle 127 is about 0.7 times adiameter of the circle 125.

The number of the rings 121, 123 and the diameters of the circles 125and 127 are adapted to the circumstances and conditions of thetransplantation to be accomplished and can be input by using thekeyboard 73 of the controller 31.

FIG. 4 shows a superimposed pattern comprising rings 121 as patternelements arranged on an outer circle 125 and rings 123 as patternelements arranged on an inner circle 127. The rings 121 and 123 are notshifted relative to each other in circumferential direction. Thus, pairsof rings 121, 123 are aligned in a radial direction relative the center115 of the pupil. This pattern of the rings 121, 123 can be used forproducing a radial suture 131 a, as this is shown on the right side ofFIG. 4. In this case, pieces 113 a of a thread are inserted in-betweenradially aligned pairs of rings 121, 123. Furthermore, the pattern 121,123 can be used for the provision of a double zigzag suture 131 b, asthis is shown on the left side of FIG. 4 by thread portions 133 b.

The eye-tracker is 63 constantly active during the surgery. Thus, thesuperimposed pattern 121, 127 is moved along with the microscopic imageof the eye, if the eye is displaced relative to the microscopy optics 5due to a force on the eye exerted by a surgical tool.

A method of introducing a toric intra-ocular lens 131 into a lenscapsule of an eye will now be described with reference to FIGS. 5 to 7.Due to its astigmatic optical effect, the toric intraocular lens 131 hasto be correctly oriented about a centre 115 of a pupil 113. During thetreatment, the pupil is dilated due to administration of a suitabledrug. In consequence, a distance between an inner rim 111 and an outerrim 107 of an iris is reduced in FIG. 7 as compared to the illustrationsof FIGS. 2 to 4.

The intra-ocular lens 131 comprises a central lens portion 133 andopposite extended peripheral portions each including a haptics 135. Thehaptics 135 are characteristic features of the intra-ocular lens whichserve as marks and can be clearly recognized in the microscopic image.

However, it is also possible that additional marks, such as lines, areprovided on the lens 131 in order to be used as orientation aids. In theillustration shown in FIG. 7, a mark produced by the pattern generator64 is superimposed with the microscopic image 101. The mark comprises acircular line 141 and a straight line 143. A diameter of the circularline 141 can be input via the keyboard by the operating surgeon or anassisting person preparing the surgery. In the situation shown in FIG. 7the diameter is selected such that it is between a diameter of the innerrim 111 and a diameter of the outer rim 107 of the iris. An orientationof the straight lines 143 about a center of the circular line 141, whichcoincides with the centre 115 of the pupil in a situation of a correctpositioning of the circular line 141, is selected such that it providesan optimal orientation of the intraocular lens 131. Thus, by using thestraight line 143, the operating surgeon can orient and position theintra-ocular lens 131 in the correct desired orientation relative to theeye.

The correct orientation of the straight line 143 that is shown in FIG. 7is adjusted as follows: FIG. 5 shows the eye before the introduction ofthe intra-ocular lens. Before the treatment, a mark 149 has beenattached to the eyeball by using a suitable tool, such as color pen orother instrument. The mark 149 is oriented in accordance with apredetermined angle α with respect to a vertical reference 151 or ahorizontal reference which has been marked before the treatment. In astep that is shown in FIG. 6 the circular line 141 and the straight line143 generated by the pattern generator 64 are superimposed with themicroscopic image. The operating surgeon or assisting person preparingthe surgery orients the straight line 143 such that the straight line143 is registered with the mark 149 attached to the eye. Then a signalis input to the pattern generator 124 to change the orientation of thestraight line 143 by an angle β such that the straight line has theorientation shown in FIG. 7. This orientation defines the correctorientation of the intraocular lens 131. Thus, it is possible to insertthe intraocular lens in an orientation that has been determined beforethe treatment relative to the marking 149. An accuracy of the insertiondoes not depend on the orientation which the eye actually has in theimage 101 during the operation. The reason is that it is difficult todefine the orientation of the eye in the image 101, since firstly theorientation of the microscopy optics 5 relative to the head of thepatient is not well defined and secondly the orientation of the eye inthe eye socket can change during the treatment.

In the embodiments illustrated with reference to FIGS. 3 and 4 thepattern elements are small circles. It is however possible to representthese pattern elements in another way such as for example by squares,lozenges, crosses, stars and similar patterns.

In the embodiment illustrated with respect to FIGS. 5 to 7, theorientation aid for the positioning of the intraocular lens is acontinuous straight line. It is, however, also possible to use othertypes of marking for orientating the intraocular lens, provided that anorientation of such marking about a center of the pupil can be adjustedwith a sufficient accuracy. For example, this marking may compriseplural portions of a straight line or portions of plural straight linescrossing each other, wherein plural of such portions may be registeredwith suitable characteristic features of the intraocular lens bycorrectly orienting the intraocular lens relative to the eye.

Furthermore, with the embodiment illustrated with reference to FIGS. 5and 6, it is possible to omit the inserted circle 141 as a partialpattern if it is to a sufficient extent ensured that the straight line143 extends through the centre 115 of the pupil. This can be achieved inparticular if the position of the inserted pattern is adjusted relativeto the image of the eye by using the eye-tracker illustrated above.

It is further possible to superimpose a pattern referred to as a TABOpattern in the art with the image, rather than merely superimposing thestraight line and circle pattern with the image. The TABO patternexhibits a plurality of single marks that are arranged relative to oneanother at the same angular distances about a center of the pattern. Themarks of the TABO pattern can be used as the orientational aid.

Apart from orienting an intraocular lens, the superimposed patterns canalso be used for other purposes where it is important to maintain adesired orientation relative to the eye. An example of such surgery is aplacement of a limbal relaxing incision (LRI).

Advantageously, the eye-tracker can also be used for placing a circularincision into the lens capsule before removing the natural lens. Forsuch purpose, the pattern generator may generate a circular pattern asillustrated, for example, in US 2004/0102799 A1. The entire contents ofthis document are incorporated herein by reference.

In the above described embodiments the position signal of theeye-tracker is used to compensate movements of the eye. This isperformed by adjusting the position of the pattern superimposed with theimage relative to the image in dependence of the position signal.Alternatively or in addition thereto, it is also possible to displacethe microscopy optics relative to the eye under surgery and independence of the position signal of the eye-tracker. For this purpose,the microscopy optics can be supported by a stand and positionedrelative to the eye. The stand may comprise an actuator for adjusting aposition of the microscopy optics relative to the eye, wherein theactuator is controlled in dependency of the position signal of theeye-tracker in order to generate the image of the eye arranged in theobject plane such that it appears in an substantially fixed positionwithin the image.

While the invention has been described with respect to certain exemplaryembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, the exemplary embodiments of the invention set forth hereinare intended to be illustrative and not limiting in any way. Variouschanges may be made without departing from the spirit and scope of thepresent invention as defined in the following claims.

1.-10. (canceled)
 11. A surgical microscopy system for performing asurgery on an eye, wherein the system comprises: a microscopy optics forgenerating an image of an object plane of the microscopy optics; apattern generator for generating a pattern superimposed with the image;and an eye-tracker for detecting a position of the eye relative to theimage and to generate a position signal indicative of the detectedposition; wherein the pattern generator is configured to change aposition of the pattern in the image responsive to the position signal.12. The surgical microscopy system according to claim 11, wherein theeye-tracker comprises an image processor which is configured todetermine a center of a contiguous dark region in the image and togenerate the position signal based on a position of the determinedcenter of a contiguous dark region.
 13. The surgical microscopy systemaccording to claim 12, wherein the image processor comprises a thresholdfilter applied to pixels of the image, such that pixels having abrightness value below a threshold are set to a first value and pixelshaving a brightness above the threshold are set to a second value. 14.The surgical microscopy system according to claim 11, further comprisinga sixth interface for inputting a size of the generated pattern relativeto the image.
 15. The surgical microscopy system according to claim 11,wherein a magnification of the microscopy optics is adjustable, andwherein the pattern generator is configured to generate the pattern suchthat a size of the pattern is proportional to the magnification of themicroscopy optics.
 16. The surgical microscopy system according to claim11, wherein the pattern comprises a ring pattern.
 17. The surgicalmicroscopy system according to claim 11, wherein the pattern generatorcomprises a projector for superimposing the pattern with a beam path ofthe microscopy optics.
 18. The surgical microscopy system according toclaim 17, wherein the projector is configured to project the pattern ina direction towards an ocular of the microscopy optics.
 19. The surgicalmicroscopy system according to claim 17, wherein the projector isconfigured to project the pattern in a direction towards the objectplane.
 20. The surgical microscopy system according to claim 11, furthercomprising a screen for displaying the image generated by the microscopyoptics and the pattern generated by the pattern generator andsuperimposed with the image.
 21. The surgical microscopy systemaccording to claim 11, further comprising a head mounted display fordisplaying the image generated by the microscopy optics and the patterngenerated by the pattern generator and superimposed with the image. 22.The surgical microscopy system according to claim 11, further comprisingan ocular for displaying the image generated by the microscopy opticsand the pattern generated by the pattern generator and superimposed withthe image.